In mass spectrometers, in order to send ions sent from a front stage into a mass analyzer, such as a quadrupole mass filter, in a rear stage while converging the ions, an ion optical element called an ion guide is used. The ion guide typically has a multipole-type configuration in which four, six, eight, or more rod electrodes having an approximately cylindrical shape are arranged apart at an interval of the same angle around an ion optical axis, and parallel to each other. In the multipole-type ion guide as described above, normally, radio-frequency voltages having the same amplitude and the same frequency, and phases inverted from each other are respectively applied to two rod electrodes circumferentially adjacent around the ion optical axis. By applying the radio-frequency voltages as described above to the respective rod electrodes, a multipole radio-frequency electric field is formed in an approximately-cylindrical space surrounded by the rod electrodes, and ions are transported while being oscillated in the radio-frequency electric field.
To meet demands for enhanced sensitivity, enhanced accuracy or other improved qualities in the mass spectrometers, it is necessary to bring the shape of equipotential lines in the radio-frequency electric field in the ion guide closer to a theoretically-derived predetermined curve, thereby improving the qualities such as ion receiving properties and ion passing properties. To this end, the accuracy in the arrangement of the respective rod electrodes needs to be improved, and in order to achieve the improvement, the present applicant proposed an ion guide having a novel configuration in Patent Literature 1. One example of the ion guide is described with reference to FIG. 9 to FIG. 13.
FIG. 9A is a side view of an ion guide unit 100, and FIG. 9B and FIG. 9C are respectively sectional views on the lines A-A′ and B-B′ in FIG. 9A. The ion guide unit 100 includes an ion guide 110 in which eight metal plates extending in the direction of an ion optical axis C are employed as electrodes, and a hollow cylindrical case 140 that encloses the ion guide 110. The respective electrodes of the ion guide 110 are arranged rotationally symmetrical so as to be apart at an interval of an angle of 45° around the ion optical axis C, with their longitudinal-side end surfaces directed toward the ion optical axis C. Here, four electrodes alternately positioned among the eight electrodes are employed as first electrodes 111, and four electrodes adjacent thereto are employed as second electrodes 112.
FIG. 10 is a perspective view of one of the first electrodes 111. In the first electrode 111, an end edge on the side of the ion optical axis C has an arc shape or a hyperbolic shape bulging toward the ion optical axis C in a sectional plane perpendicular to the ion optical axis C. Further, the end edge on the side of the ion optical axis C is slightly inclined with respect to the ion optical axis C so as to become slightly apart from the ion optical axis C as an ion travels (rightward in FIG. 9C and FIG. 10). Because of the inclination, the intensity of the multipole electric field is smaller toward the outlet side of the ion guide 110, thereby decelerating flying ions. The other three plate electrodes of the first electrode 111, and the four electrode plates of the second electrodes 112 adjacent thereto also have the same shape.
The case 140 includes a tubular section 141 that encloses the first electrodes 111 and the second electrodes 112, a first support section 142 that is attached to one end portion of the tubular section 141 to support one end surfaces (left-side end surfaces in FIG. 9C) of the respective electrodes, a second support section 143 that is attached to the other end portion of the tubular section 141, and a disk spring fixing section 144 that fixes a disk spring 130 as shown in FIG. 11A by sandwiching the disk spring 130 between the disk spring fixing section 144 and the second support section 143. The first support section 142 and the second support section 143 are made of insulators such as ceramics, plastics or the like, and an opening for allowing ions to pass therethrough is provided in the center. A cylindrical through hole is provided in the second support section 143 at a position corresponding to each of the electrodes.
The disk spring 130 shown in FIG. 11A is made of metal, and includes a ring-shaped frame portion 131 and eight spring portions 132 working as cantilever springs projecting inward from the frame portion 131. The spring portions 132, each having a T shape with the head inward, are arranged so that the heads are close, but without contacting, to each other.
A thin plate 150 made of metal as shown in FIG. 11B is placed on a surface supporting the electrodes in the first support section 142. The thin plate 150 includes a ring-shaped frame portion 151 and four metal contacts 152 projecting inward from the frame portion 151. In the thin plate 150 placed on the first support section 142, the positions of the metal contacts 152 correspond to the positions of the first electrodes 111. Accordingly, the thin plate 150 contacts only the first electrodes 111, and does not contact the second electrodes 112.
FIG. 12 is a plan view of a state in which the disk spring 130 and the disk spring fixing section 144 are removed from the end portion on the side of the second support section 143 of the ion guide unit 100. Insulating spacers 121 made of insulators are inserted into four holes corresponding to the first electrodes 111, and conducting spacers 122 made of conductors are inserted into four holes corresponding to the second electrodes 112, as to the eight through holes provided in the second support section 143. The respective spacers are cylindrical members having the same length, which sets one end of the spacer to slightly project from the surface of the second support section 143 when the other end is in contact with the electrode.
FIG. 13 is a partial plan view of a state in which the disk spring 130 and the disk spring fixing section 144 are attached to the ion guide unit 100 shown in FIG. 12. The disk spring 130 is arranged such that the right and left ends close to each other of the adjacent spring portions 132 press the projecting portion of one insulating spacer 121 or one conducting spacer 122. Accordingly, the disk spring 130 is insulated from the first electrodes 111 by the insulating spacers 121, and electrically connected to the second electrodes 112 via the conducting spacers 122.
In the ion guide unit 100 having the above configuration, the spring portions 132 of the disk spring 130 press the first electrodes 111 and the second electrodes 112 toward the first support section 142 via the insulating spacers 121 or the conducting spacers 122. Accordingly, the respective electrodes 111 and 112 are sandwiched between the disk spring 130 and the first support section 142 from both sides and thereby fixed. At this point, end surfaces of the first electrodes 111 are in contact with the insulating spacers 121 or the metal thin plate 150, and end surfaces of the second electrodes 112 are in contact with the conducting spacers 122 or the second support section 143 made of an insulator. A voltage VDC+v·cos ωt in which a radio-frequency voltage v·cos ωt is superimposed on a direct current voltage VDC is applied to the first electrodes 111 via the thin plate 150, and a voltage VDC−v·cos ωt in which a radio-frequency voltage of inverted phase (i.e., phase shifted by 180°) is superimposed on the same direct current voltage is applied to the second electrodes 112 via the disk spring 130 and the conducting spacers 122 from a voltage application section (not shown in the drawing). Accordingly, a multipole radio-frequency electric field is formed in the space surrounded by the edge end surfaces of the eight electrodes 111 and 112, and ions introduced therein are converged.
Since the end edges of the eight electrodes 111 and 112 facing the ion optical axis C have an arc shape or a parabolic shape convex toward the ion optical axis C in a plane perpendicular to the ion optical axis C, an electric field whose equipotential lines are shaped along the curve is generated in the vicinity of the electrodes 111 and 112. Thus, an electric field nearly an ideal state can be formed in the space surrounded by the end surfaces of the respective electrodes 111 and 112.
Recent mass spectrometers tend to have a complicated configurations where, for example, a plurality of multipole-type ion guides as described above are used. In a liquid-chromatograph tandem quadrupole mass spectrometer described in Non Patent Literature 1, for example, a two-stage octupole-type ion guides are provided between an ion source and a first-stage quadrupole mass filter, and a quadrupole-type ion guide is disposed within a collision cell. That is, a plurality of ion guides having different number of poles are used in an apparatus. In conventional mass spectrometers, ion guides having different number of poles as described above have respective configurations different from each other. For example, when the above ion guide unit 100 is used, it is necessary to change not only the number of the metal plate electrodes, but also the shape of the members for holding the metal plate electrodes, such as the first support section 142, the second support section 143 and the disk spring 130, according to the number of poles. If, in the mass spectrometer using a plurality of ion guides as described above, ion guides having the same structure can be used, it is advantageous in reducing the cost.